DESCRIPTION
This invention relates to electrical equipment, and in particular to surge arrestors. The invention also relates to a packing arrangement.
Surge arrestors are used in the electrical power industry for limiting the magnitude of surge voltages, due for example to lightning strikes, between two points of a power system at different electrical potentials. Typically, a surge arrestor is of generally cylindrical configuration and may comprise a non-linear resistor arrangement formed from a plurality, that is to say two or more, of metaloxide, for example zinc oxide, varistor blocks stacked end-to-end, having a conductive plate, or other surface, at each end to form a pair of electrodes. The resistor arrangement and electrodes are mounted within and retained by a housing. The housing is of insulating material, advantageously having good resistance to tracking, that is to say good resistance to the formation of carbonaceous paths along its surface, and advantageously has good resistance to environmental contaminants such as water, salts and acids. Advantageously, the outer surface has a shedded or convoluted configuration, thereby increasing the creepage path length end-to-end. Porcelain is a good and commonly-used material for surge arrestor housings, being disclosed for example in UK Patent Publication No. 2050719A, but although porcelain has very good electrical and environmental properties, it does have the disadvantage of resulting in a relatively heavy structure. UK Patent Publication No. 2073965B discloses a surge arrestor in which the housing is provided by an anti-tracking heat shrinkable polymeric material, which provides a considerable weight saving with respect to procelain. However, a porcelain housing serves the further function of being strong enough to retain the surge arresting blocks mechanically. In this latter respect, it is pointed out that the electrical surge that can pass through the surge arrestor in operation, and particularly in an overload condition, can give rise to severe mechanical forces that tend to urge the surge arresting elements longitudinally apart. This tendency is overcome in conventional porcelain surge arrestors by having a spring urge the elements together, the arrangement of elements and spring being held in compression between the electrodes by fixing them to the housing, which in the case of procelain is strong and rigid enough to withstand the reaction forces. A porcelain housing is consequently subject to explosive shattering in extreme cases, even despite the provision of a rupturable pressure-relieving diaphragm, a problem which does not arise with a surge arrestor having a polymeric housing, which is thus inherently safer. A polymeric housing, however, particularly when the polymeric material is made thin enough to be heat-recoverable, does not have the mechanical strength of procelain. In one known configuration of a surge arrestor having a polymeric housing, the surge arresting elements are formed as rings rather than solid discs, and a fibreglass rod extends axially through the rings and is threaded into an electrode at each end of the stack of discs so as longitudinally to compress them. However, this configuration gives rise to an air-filled channel extending between the electrodes, which produces mechanical weakness in the surge arrestor and also is undesirable electrically.